Polymer Journal, Vol. 37, No. 5, pp. 333–339 (2005) A New Trend in Radiation Vulcanization of Natural Rubber Latex with a Low Energy Electron Beam y yy MD. Emdadul HAQUE,1; , Keizo MAKUUCHI,1 Hiroshi MITOMO,2 Fumio YOSHII1 and Kenichi IKEDA1 1Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki 370-1292, Japan 2Department of Biological and Chemical Engineering, Faculty of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515, Japan (Received April 30, 2004; Accepted February 11, 2005; Published May 15, 2005) ABSTRACT: The natural rubber latex (NRL) was radiation vulcanized under a pilot plant of low energy electron accelerator. Accelerating voltage and maximum beam current of this accelerator are 250 kV and 10 mA respectively. Irradiation on NRL was carried out in a reaction vessel with constant stirring. In order to obtain a suitable setting of experiment for radiation vulcanization with the plant, the parameters such as defoamer concentration, irradiation time, volume of latex, beam current, latex concentration etc. were optimized by varying the individual parameter at a constant set of the other variables. Radiation vulcanization accelerators (RVA) were normal butyl acrylate (n-BA) and nonane-diol-diacrylate (NDDA). Twenty minutes irradiation time is enough to vulcanize 14 L of latex when 5 phr RVA (any type) are used. Maximum tensile strength of the radiation vulcanized NRL films obtained with 5 phr of NDDA or n-BA is 26 MPa. About 30 MPa tensile strength was obtained from the rubber film by irradiating 25% diluted latex for 20 min. [DOI 10.1295/polymj.37.333] KEY WORDS Electron Beam / NDDA / RVA / NRL / Tensile Strength / Radiation method for the vulcanization of natural of NRL was carried out with 3 MeV electron acceler- rubber latex (NRL) emerged in late fifties and got ator (Dynamitron) using a cylindrical stainless steel popularity for some distinct advantages over the con- reaction vessel of 3.2 L capacity having outer jacket ventional one especially for its approach to cleaner en- and propeller type stirrer.4 For homogeneity of vul- vironment. Radiation vulcanization of NRL is carried canization higher stirring speed was suggested. out with electron beam (EB) from accelerator machine Attention was drawn to the low energy electron ac- side by side with gamma rays from Cobalt-60. But celerator because it does not require any special build- vulcanization with EB did not become so popular be- ing with thick bio-shielding. NRL was irradiated with cause of its low penetration into the NRL. There are the accelerating voltage of 300 kV varying the beam also possibilities of over dosing and rise of tempera- current, stirring speed, etc.5 The effect of beam cur- ture of latex during irradiation. Efforts were made to rent on tensile strength (Tb) of RVNRL film was overcome these critical drawbacks. The radiation vul- not found but with higher stirring speed shorter vul- canization of NRL without radiation vulcanization ac- canization time could be achieved. The radiation vul- celerator (RVA) have been studied in China by using canization was also carried out with electron acceler- a van de Graaff electron accelerator (2 MeV, 0.15 mA) ator CB 250/15/180L having drum type reaction with constant stirring to ensure homogeneity.1 A pilot vessel and was reported to be good enough.6 plant of linear electron accelerator having the energy NRL can be vulcanized by ionizing radiation with- of 6 MeV was installed in France to irradiate latex out any accelerator or additive.7,8 But the required continuously in a specially designed vessel connected dose is very high. So RVA is used to reduce the vul- to a circulating pump.2 It was reported that the result- canization dose for economic reason.9,10 The acceler- ing radiation vulcanized NRL (RVNRL) and its film ating mechanism of monofunctional monomers were were of good properties. In Germany NRL was irradi- investigated using liquid poly(isoprene) and 2-ethyl- ated with a Dynamitron accelerator (1.5 MeV, 25 mA) hexylacrylate as a model system.11 It was indicated by flow of latex on metal slope using a polyfunctional that the cross-linking of rubber takes place through monomer as RVA.3 At Takasaki Radiation Chemistry the graft polymerization of acrylate onto liquid rub- Research Establishment, Japan radiation vulcanization ber. Till now normal butyl acrylate (n-BA) is being yPresent address: Institute of Nuclear Science & Technology, Bangladesh Atomic Energy Commission, P.O. DEPZ, Ganakbari, Savar, Dhaka, Bangladesh yyTo whom correspondence should be addressed (Fax: 880-2-7701337, E-mail: [email protected]; [email protected]). 333 MD. E. HAQUE et al. used as an effective RVA.12–17 But it has bad smell and enhances the coagulation of latex.18,19 The bad smell of RVA pollutes the working environment and may jeopardize the aim of commercialization of the process. Moreover the latex particles from RVNRL come in contact with skin. So the RVA should have minimum Primary Skin Irritation Index (P.I.I.). Nonane-diol-diacrylate (NDDA), CH2=CH–COO– (CH)9–OOC–CH=CH2, has no smell and relatively low P.I.I. of 1.9. The accelerating efficiency of a di- functional monomer, NDDA for radiation vulcaniza- tion of NRL was investigated to replace n-BA.21 The NDDA mixed latex remains in a very good phys- ical state (in terms of color and viscosity) up to several days. Considering these advantages it was selected as a RVA. n-BA was also used for comparison. In order to produce RVNRL with suitable strength and quality for the production of dipped goods, especially hand gloves, various schemes were adopted in irradiating latex with electron beam pilot plant. The following features are considered advantageous for EB. (1) Small size, (2) No special building with thick bio- shielding is required, (3) Self-shielded, (4) The elec- Figure 1. Front view of low energy electron accelerator pilot tron beam is stopped with the shut down of the power plant for RVNRL. source, (5) Low irradiation cost (expected). The pa- rameters of the pilot plant were optimized in order to obtain a suitable setting of experimental for radia- tion vulcanization of NRL under EB. EXPERIMENTAL Main Features of EB Pilot Plant for Radiation Vul- canization of NRL The pilot plant (Figure 1) for radiation vulcaniza- tion of NRL at Takasaki Radiation Chemistry Re- search Establishment, Japan has a reaction vessel with the capacity of 18 L latex to irradiate at a time. The accelerating voltage and beam current of the plant were 250 kV and 10 mA respectively. The length and width of the beam window were 20 and 6 cm respectively. The latex irradiation vessel (Figure 2) under EB was a cylindrical stainless steel vessel (È ¼ 29 cm and H ¼ 30:5 cm) containing four baffle plates onto Figure 2. Reaction vessel of low energy electron accelerator the inner wall at 90 intervals and fitted with a pro- (EB). peller type stirrer and outer jackets. The vessel was covered with a cooling plate having titan film cally with the help of the right and left turns of a key. (thickness ¼ 0:0015 cm) window (20 Â 8 cm2). The The vessel has an outlet at the bottom to channel out reaction vessel was installed on a basement under the irradiated latex and to remove the waste-water af- the beam window. Backward and forward movements ter washing the vessel. The irradiation was carried out of the vessel together with the basement were done after setting the vessel at the inbuilt set top up position manually. The vessel was pulled forward for feeding and shutting up the plant door. the latex and after completion of feeding it was again replaced to the position by pushing backward. The up- Cover of the Vessel ward and downward movements were done mechani- The reaction vessel has a cooling cover with titan 334 Polym. J., Vol. 37, No. 5, 2005 A New Trend in Radiation Vulcanization film window. This cover was not sufficient to keep swelling. The cross-link density was calculated from cool the vessel. Titan film is very expensive. So alu- swelling ratio using Flory and Rehner equation.20 minum foil was used as the cover of the vessel. It was fairly simple and easy to use and also very cheap RESULTS AND DISCUSSION in comparison to titan film. There was no difference in quality and strength of the RVNRL films obtained by Optimization of Defoamer Concentration irradiating the latex covering with aluminum foil to By vigorous stirring of the latex many bubbles were that obtained with titan film. formed and created problem during irradiation. So a defoamer (BYK022) was used to suppress the bubble Preparation of Emulsion of RVA formation. To optimize the defoamer concentration The emulsion of RVA was prepared by mixing the varying quantities (0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 emulsifier (Emulgen-420Ô, Nippon Oil and Fats Co., parts/100 rubber (phr)) of defoamer were added to Ltd., Japan) with RVA and water. The ratio of emul- the 5 phr NDDA impregnated latex. The mixture sifier, water and RVA (monomer) = 1:99:100. It was was stirred with low to high speed for 20 min with a stirred for 1–2 h using a ball mixer. magnetic stirrer. It was found that 0.2 phr concentra- tion of defoamer was sufficient to suppress the bubble Irradiation of NRL under EB formation when latex was stirred at 360 rpm. The A high ammonia latex concentrate (Microtex, Ma- maximum rotational speed of the stirrer in the EB laysia) was used after diluting with 1% aqueous reactor vessel was 360 rpm.